Comparison of damage to live v. euthanized Atlantic salmon Salmo salar smolts from passage through an Archimedean screw turbine.

This study assessed the usefulness of passing euthanized Atlantic salmon Salmo salar smolts through an Archimedean screw turbine to test for external damage, as compared with live, actively swimming smolts. Scale loss was the only observed effect. Severe scale loss was 5·9 times more prevalent in euthanized turbine-passed fish (45%) than the live fish (7·6%). Additionally, distinctive patterns of scale loss, consistent with grinding between the turbine helices and housing trough, were observed in 35% of euthanized turbine-passed smolts. This distinctive pattern of scale loss was not seen in live turbine-passed smolts, nor in control groups (live and euthanized smolts released downstream of the turbine), which suggests that the altered behaviour of dead fish in turbine flows generates biased injury outcomes.

The influence of aquaculture unit proximity on the pattern of Lepeophtheirus salmonis infection of anadromous Salmo trutta populations on the isle of Skye, Scotland.

A total of 230 anadromous Salmo trutta (brown trout) were sampled in five sheltered coastal fjords (or sea lochs) on the Isle of Skye, Scotland, U.K., in 2016 at varying distances from active Atlantic salmon Salmo salar farms. Statistical models were developed to investigate potential correlations between salmon lice Lepeophtheirus salmonis burdens on S. trutta hosts and their proximity to S. salar farm cages. Significant correlations were found between lice burdens and fish fork length and proximity to the nearest S. salar farm. The probability of the presence of L. salmonis on fish hosts increased with fish host size and with distance from the nearest S. salar farm, but total lice burdens were highest in fish sampled near S. salar farms and decreased with distance. The proportion of different life-cycle stages of L. salmonis were also dependent on S. salar farm proximity, with higher juvenile lice numbers recorded at sites near S. salar farm cages. These results highlight the complexity of the relationship between S. trutta and L. salmonis infections on wild fish and emphasize the requirement of further research to quantify these effects to better inform conservation and management strategies, particularly in areas of active S. salar farm facilities.

The effect of foraging and ontogeny on the prevalence and intensity of the invasive parasite Anguillicola crassus in the European eel Anguilla anguilla.

Infection patterns of the invasive Anguillicola crassus nematode were investigated in a population of the European eel Anguilla anguilla where parasite invasion is very recent, Loch Lomond, Scotland. Intensity levels of the parasite were associated with differences in fish ontogeny and trophic ecology. Although eels foraged on both fish and invertebrates, individuals which were smaller and fed on invertebrates (>70% contribution to diet) were found to contain a greater number of swim bladder parasites compared to larger eel with a predominance of fish (>60% contribution) in their diet. Within affected fish, a significant negative relationship was found between fish length and parasite intensity, with smaller individuals having higher parasite intensity than larger individuals. This study indicates that food intake and infection risk are linked in this recently infected host-parasite system. From a management perspective increasing our understanding of how infection intensity and repeated exposure is linked to resource use in an ecosystem is important for the future management of this endangered species in Europe.

Lake bathymetry and species occurrence predict the distribution of a lacustrine apex predator.

This study examined the abiotic and biotic characteristics of ecosystems that allow expression of a life history called ferox trout, the colloquial name given to brown trout Salmo trutta adopting a piscivorous life history strategy, an apex predator in post-glacial lakes in northern Europe. One hundred and ninety-two lakes in Scotland show evidence of currently, or historically, supporting ferox S. trutta; their presence was predicted in logistic models by larger and deeper lakes with a large catchment that also support populations of Arctic charr Salvelinus alpinus.

Does size matter? A test of size-specific mortality in Atlantic salmon Salmo salar smolts tagged with acoustic transmitters.

Mortality rates of wild Atlantic salmon Salmo salar smolts implanted with acoustic transmitters were assessed to determine if mortality was size dependent. The routinely accepted, but widely debated, '2% transmitter mass: body mass' rule in biotelemetry was tested by extending the transmitter burden up to 12·7% of body mass in small [mean fork length (LF ) 138·3 mm, range 115-168 mm] downstream migrating S. salar smolts. Over the short timescale of emigration (range 11·9-44·5 days) through the lower river and estuary, mortality was not related to S. salar size, nor was a relationship found between mortality probability and transmitter mass: body mass or transmitter length: LF ratios. This study provides further evidence that smolt migration studies can deviate from the '2% rule' of thumb, to more appropriate study-specific measures, which enables the use of fishes representative of the body size in natural populations without undue effects.

Freshwater and coastal migration patterns in the silver-stage eel Anguilla anguilla.

The unimpeded downstream movement patterns and migration success of small female and male Anguilla anguilla through a catchment in north-west Europe were studied using an acoustic hydrophone array along the River Finn and into the Foyle Estuary in Ireland. Twenty silver-stage A. anguilla (total length, LT , range: 332-520 mm) were trapped 152 km upstream from a coastal marine sea-lough outlet and internally tagged with acoustic transmitters of which 19 initiated downstream migration. Migration speed was highly influenced by river flow within the freshwater (FW) compartment. Anguilla anguilla activity patterns were correlated with environmental influences; light, tidal direction and lunar phase all influenced the initiation of migration of tagged individuals. Migration speed varied significantly between upstream and lower river compartments. Individuals migrated at a slower speed in transitional water and sea-lough compartments compared with the FW compartment. While 88·5% survival was recorded during migration through the upper 121 km of the river and estuary, only 26% of A. anguilla which initiated downstream migration were detected at the outermost end of the acoustic array. Telemetry equipment functioned efficiently, including in the sea-lough, so this suggests high levels of mortality during sea-lough migration, or less likely, long-term sea-lough residence by silver A. anguilla emigrants. This has important implications for eel management plans.

Inter and intra-population phenotypic and genotypic structuring in the European whitefish Coregonus lavaretus, a rare freshwater fish in Scotland.

This study revealed between-lake genetic structuring between Coregonus lavaretus collected from the only two native populations of this species in Scotland, U.K. (Lochs Eck and Lomond) evidenced by the existence of private alleles (12 in Lomond and four in Eck) and significant genetic differentiation (FST = 0·056) across 10 microsatellite markers. Juvenile C. lavaretus originating from eggs collected from the two lakes and reared in a common-garden experiment showed clear phenotypic differences in trophic morphology (i.e. head and body shape) between these populations indicating that these characteristics were, at least partly, inherited. Microsatellite analysis of adults collected from different geographic regions within Loch Lomond revealed detectable and statistically significant but relatively weak genetic structuring (FST = 0·001-0·024) and evidence of private alleles related to the basin structure of the lake. Within-lake genetic divergence patterns suggest three possibilities for this observed pattern: (1) differential selection pressures causing divergence into separate gene pools, (2) a collapse of two formerly divergent gene pools and (3) a stable state maintained by balancing selection forces resulting from spatial variation in selection and lake heterogeneity. Small estimates of effective population sizes for the populations in both lakes suggest that the capacity of both populations to adapt to future environmental change may be limited.

Local scale, coastal currents influence recruitment to freshwater populations in the European eel Anguilla anguilla: a case study from the Isle of Man.

This study examines juvenile Anguilla anguilla (<30 cm) abundance in five study catchments on the Isle of Man. Preliminary results suggest that juvenile abundance is negatively correlated with increasing coastal current speed at river mouth entry (P < 0·05). These findings indicate that at least under some circumstances, tidally driven coastal currents may influence recruitment to freshwater habitats; therefore, it is presumed that high coastal current speed at the entry to river mouths may reduce the likelihood of freshwater entry.

Morphological abnormalities in a population of Lampetra planeri, with a short review of petromyzontid teratologies.

Reported here are several deviations (n = 15; 1·96%) from typical morphology in a large sample (n = 767) of European brook lamprey Lampetra planeri from a single population in the Loch Lomond catchment; this includes one specimen bearing a true anal fin. A brief review of petromyzontid teratology is provided.

Surface feeding and aggressive behaviour of diploid and triploid brown trout Salmo trutta during allopatric pair-wise matchings.

Diploid and triploid brown trout Salmo trutta were acclimated for 6 weeks on two feeding regimes (floating and sinking). Thereafter, aggression and surface feeding response were compared between pairs of all diploid, all triploid and diploid and triploid S. trutta in an experimental stream. In each pair-wise matching, fish of similar size were placed in allopatry and rank was determined by the total number of aggressive interactions recorded. Dominant individuals initiated more aggression than subordinates, spent more time defending a territory and positioned themselves closer to the surface food source (Gammarus pulex), whereas subordinates occupied the peripheries. In cross ploidy trials, diploid S. trutta were more aggressive than triploid, and dominated their sibling when placed in pair-wise matchings. Surface feeding, however, did not differ statistically between ploidy irrespective of feeding regime. Triploids adopted a sneak feeding strategy while diploids expended more time defending a territory. In addition, we also tested whether triploids exhibit a similar social dominance to diploids when placed in allopatry. Although aggression was lower in triploid pairs than in the diploid and triploid pairs, a dominance hierarchy was also observed between individuals of the same ploidy. Dominant triploid fish were more aggressive and consumed more feed items than subordinate individuals. Subordinate fish displayed a darker colour index than dominant fish suggesting increased stress levels. Dominant triploid fish, however, appeared to be more tolerant of subordinate individuals and did not display the same degree of invasive aggression as seen in the diploid and diploid or diploid and triploid matchings. These novel findings suggest that sterile triploid S. trutta feed similarly but are less aggressive than diploid trout. Future studies should determine the habitat choice of triploid S. trutta after release and the interaction between wild fish and triploids during the breeding season prior to utilization of triploids as an alternative management strategy within freshwater fisheries.